1. Field of the Invention
This invention relates to a projecting apparatus wherein a plurality of video sources from respective video images which are projected through focusing means and presented as a compound image on a screen.
2. Description of the Prior Art
A typical color video projector is provided with three cathode ray tubes which form a red video image, a green video image and a blue video image. These three images are projected through respective focusing means such as lenses and are compounded and mixed on a screen.
Typically, the cathode ray tubes which form the green and red video images are inclined in a horizontal direction so that the central axes of the tubes intersect at the center of the screen and the cathode ray tube which forms the blue image is vertically oriented so that its image is facing in the vertical direction and is reflected, as by a semi-reflecting mirror, so as to be projected on the screen. In this manner, the three images are compounded and mixed on the screen. However, because of the described inclination or orientation of the cathode ray tubes and their respective lenses, the images projected on the screen are deformed, typically in the sense to distort a rectangular screen-shape into a trapezoidal form, with the red and blue trapezoid images being oriented in one direction and the green trapezoid image being oriented in the opposite direction. As a result, there occurs a misalignment of the respective color images on the screen.
In order to correct this deformation of the color images and consequent misalignment thereof, one known system uses conventional cathode ray tubes which are provided with expensive electronic correcting means which form reversely deformed video signal sources to cancel out the projected optical deformations. Another projection system arranges the projecting lenses so that the principal axes thereof are parallel to each other and symmetrically arranged about a normal to the screen and the central axes of the cathode ray tubes are epaxially displaced from the principal axes of the respective ones of the projecting lenses. However, such a system provides an uneven distribution of brightness of the different color images projected on the screen. As a result, it is necessary to use relatively complex and expensive circuitry to compensate for these variations in brightness.